Mrs Roberts, 37, objected to Neon receiving radiotherapy because she feared it could cause him to suffer serious side effects such as stunted growth and infertility, which would prevent her from having grandchildren.

But three weeks ago a High Court judge dismissed her case after doctors advised Neon would die within three months if he was denied the “gold standard” treatment.

He began his first round of radiotherapy yesterday after completing a course of play therapy and a practice run on Wednesday to prepare him.

Sources said he was “doing very well”.

While Mrs Roberts had previously suggested she would appeal the High Court’s decision, no such appeal was made by her lawyers before Neon’s treatment began.

But she has insisted she remained “100%” against radiotherapy being given to her son.

She said: “I have seen pictures of Neon having the play therapy. It is heartbreaking.”

Legal fight: Sally Roberts insisted she is not a "bonkers" mum (Image: PA)

Neon was taken to a top hospital, which has not been identified, by his dad Ben, 34.

He has been living with his father since his mum was found by police on the run with him.

It is understood the first round of treatment began without any immediate medical problems as Mr Roberts, of Knightsbridge, West London, remained at his son’s bedside.

The medical team has spent the past fortnight preparing him for the “complex procedures”.

Treatment is expected to last several weeks.

A source said: “He is doing very well. The doctors have done an excellent job explaining it to him. It has been made clear to him.”

During a series of legal hearings last month, a judge was told by one of Neon’s doctors, an unnamed leading British paediatric oncologist, that radiotherapy was the best chance to beat the life-threatening disease.

Surgeons say he would die within three months without it.

They have grown increasingly concerned that his chances of survival have been damaged because of delays caused by his mum’s legal battle.

Neon had the cancerous brain tumour, called medulloblastoma, first removed in October.

He had a second operation to remove more cancerous cells from his brain last month, in a procedure also opposed by his Brighton-based mum.

Mrs Roberts has previously insisted she is not a “bonkers mother” because she opposed her “vibrant” son getting radiotherapy treatment for his tumour.